The lawyer heading the defense for the alleged Boston Marathon bomber in federal court in Boston is well-versed in capital cases and the long, emotionally charged process that would come if federal prosecutors seek the death penalty, according to legal observers and her own colleagues.

Judy Clarke, a former public defender now working as a private lawyer in San Diego, has been tapped to represent Dzhokhar Tsarnaev, the 19-year-old from Cambridge charged in the bombings that killed three people and injured more than 260. Three other public defenders from Boston have also been assigned to the case, including one who speaks Tsarnaev’s native Russian.

Cases involving the death penalty allow for the appointment of an outside lawyer “learned in the law applicable to capital cases,” according to court records, and Clarke has been at the center of some of the most high-profile cases that the country has seen in recent times — cases that often include emotionally charged public calls for the death penalty.

“It’s a huge challenge, and if anyone is up for it, it’s Judy Clarke,” said Quin Denvir, a former public defender who has worked with her. “They’re really difficult cases. The client’s life is at stake, it’s a huge moral battle.”

Already, the defense team won a request to have photos of Tsarnaev taken to show his state of health after his violent escape and then capture by police, and they could be used as evidence to argue against the death penalty.

Denvir said Clarke is known not only for her experience completing the research and tedious paperwork required of such complex cases, but also for her legal wit. Clarke, very low-key, does not engage the news media. Instead, she is calculating in her cases.

Above all, Denvir and others said, she has the passion of a public defender — to represent clients who are otherwise seen by the public to be monsters: She opposes the death penalty, not only as a lawyer but in her political beliefs.

Clarke most recently defended Jared Loughner, the man behind the Tucson shooting rampage in 2011 that killed six people and injured a dozen others, including Representative Gabrielle Giffords, his target.

She defended Theodore John “Ted” Kaczynski, known as the Unabomber, for the nationwide bombing campaign that killed three people. She also represented Atlanta Olympic bomber Eric Rudolph and Susan Smith, the South Carolina woman who murdered her three children.

In each of those cases, the defendants were found guilty, but Clarke helped them escape the death penalty, and it will undoubtedly be her priority as she embarks on the Tsarnaev case, according to legal observers who described an in-depth, complex, and drawn-out process in capital cases.

“She has a great rapport with clients, she takes a long time trying to understand them, where they’re coming from, explaining what she’s trying to do, build a sense of trust and a way to work together,” Denvir said. “It’s not just a question of a political, or moral stand, but as a lawyer you help fight for that person’s life.”

One tale often told by her colleagues: After South Carolina sought to ban payments to outside public defenders, following the Smith case, Clarke returned the $82,944 fee a judge approved for her defense of Smith, asking that the funds be used to defend indigent defendants in other cases.

“She gets called upon and she feels, if she can help out, she has an obligation,” Denvir said.

The process in capital cases can be quite complex, according to legal observers. Attorney General Eric Holder will ultimately decide whether to seek the death penalty, based on input from defense attorneys and also from US Attorney Carmen M. Ortiz of Massachusetts. Ortiz’s decision will not be made public, and a spokeswoman for her office would only say, “The US attorney plans to rely on the advice and counsel of senior management in the office.”

Throughout the process, however, defense lawyers can give their input, and Clarke and others on the defense team will probably cite Tsarnaev’s age, his lack of a criminal history, and his older brother’s reportedly negative influence on him as reasons why he should not be executed. The older brother, Tamerlan Tsarnaev, was said by friends to be the more radical of the two. He was killed in a shoot-out with police and reportedly run over by a car driven by the younger brother.

Since 1988, when federal death penalty laws were first passed, federal prosecutors have authorized seeking the death penalty for 492 defendants. However, a majority of them avoided trial by negotiating a plea, or because the government dropped charges against them or dropped the death penalty, according to data compiled by the Federal Death Penalty Resource Counsel.

In the 215 times that a jury had to choose, they chose death in 72 instances, or 34 percent of the time, according to the Resource Counsel. Ultimately, three people were executed, while other cases are on appeal or defendants died in prison awaiting appeal.

In Massachusetts, jurors in 2002 chose death in the case of Gary Lee Sampson, who went on a bloody rampage over several days and killed three people. A federal judge overturned the jury’s decision, however, and ordered a new sentencing trial after finding that one of the jurors lied during the selection process. That decision is under appeal.

Federal prosecutors authorized the death penalty two other times in Massachusetts. In the case of Darryl Green and Branden Morris, two Dorchester gang members, federal prosecutors dropped the charges and the men were tried in state court.

In the case of Kirsten Gilbert, the former nurse was convicted in 2001 of administering lethal injections to patients, causing them to have heart attacks, a jury chose a life sentence rather than the death penalty.

David Hoose, a lawyer who has tried several death penalty cases, and who is set to try a capital case in Rhode Island in the summer, said in an interview that it takes a certain conviction for someone to want to specialize in death penalty cases, and that Clarke has emerged as a “go-to” person to represent the most high-profile defendants.

“It’s an opportunity to work on cases where the stakes are so much higher, and the defendant needs someone to take on that type of burden,” said Hoose, of Sasson, Turnbull, Ryan & Hoose of Northampton.